== Why is this an issue?

In Kotlin, nullability is a part of the type system. This means that all types could be used in 2 ways. `T` - as non-nullable and `T?` as nullable. When you want to access properties or instance methods of a nullable type, you need to handle the potential null value. However, while accessing a non-nullable type, you don't need to test nullability as the compiler knows that the value can never be `null`. So all the nullability checks on the non-nullable types are considered code smells.

Here is an example of non-nullable variable. `s` is of a type `String` and cannot be `null`.

[source, kotlin]
----
val s: String = ""
----

Here is an example of nullable variable. Nullable variables are declared by using the `?`.

[source, kotlin]
----
val s: String? = null
----


== How to fix it

Avoid using null checks on non-nullable variables.

=== Code examples

If your variable type is non-nullable, any null checks are redundant. For example, `if (s == null) {}`, `requireNotNull(s)` and `checkNotNull(s)` can be dropped from your code.


==== Noncompliant code example

[source, kotlin,diff-id=1,diff-type=noncompliant]
----
val s: String = ""
if (s != null) { doSomething() } // This statement is always true
----

==== Compliant solution

[source, kotlin,diff-id=1,diff-type=compliant]
----
val s: String = ""
doSomething()
----

==== Noncompliant code example

[source, kotlin,diff-id=2,diff-type=noncompliant]
----
val s: String = ""
if (s == null) { doSomething() } // This statement is always false.
----

==== Compliant solution

[source, kotlin,diff-id=2,diff-type=compliant]
----
val s: String = ""
----

==== Noncompliant code example

[source, kotlin,diff-id=3,diff-type=noncompliant]
----
val s: String = "ABC"
return s ?: "" // This ?: is useless.
----

==== Compliant solution

[source, kotlin,diff-id=3,diff-type=compliant]
----
val s: String = "ABC"
return s
----

==== Noncompliant code example

[source, kotlin,diff-id=4,diff-type=noncompliant]
----
val s: String = ""
s!!.doSomething() //You do not need to assert that S is non-null.
----

==== Compliant solution

[source, kotlin,diff-id=4,diff-type=compliant]
----
val s: String = ""
s.doSomething
----


== Resources

=== Documentation

* https://kotlinlang.org/docs/null-safety.html#nullable-types-and-non-null-types[Kotlin Documentation - Null Safety]
* https://kotlinlang.org/docs/strings.html[Kotlin Documentation - Strings]

=== Articles & blog posts

* https://blog.logrocket.com/complete-guide-null-safety-kotlin/[A complete guide to null safety in Kotlin]